Pistons used heretofore in reciprocating engines are subjected to a variety of destructive forces, including: high thermal stress (particularly about the upper face of the piston), high temperature friction forces which wear the piston zone immediately above the piston rings, and high inertia forces resulting from the density of the piston material. It is now common to use an aluminum based piston to reduce weight inertia, but such aluminum is not high in wear resistance and is weak at high temperatures requiring insulation to protect it in high performance applications.
The state of the art has embedded, by friction, welding, annular wear resistant rings (of steel, bronze, or silicon alloys) in the upper shoulder of aluminum pistons to reduce high temperature frequency friction wear (see U.S. Pat. No. 3,596,571). However, such composite structure fails to reduce or eliminate high temperature thermal stress and is devoid of an insulative member.
The state of the art has coated ceramic to an aluminum piston to improve the insulating quality of the hot top of the piston (see U.S. Pat. No. 4,245,611). However, such coating gives insufficient insulation unless extremely thick, which fact then contributes to cracking and differential thermal expansion problems associated with the supporting aluminum piston. The art has also sintered a graduated powder mixture of silicon carbide and iron to the top face of an iron piston in an effort to insulate the base portion of the piston from the hot zone of the engine (see U.S. Pat. No. 2,657,961). Unfortunately, silicon carbide has a thermal conductivity higher than iron, even in a porous condition, and thus prevents obtaining a significant improvement in temperature insulation of the piston. This art fails to suggest a way of avoiding thermal stress in aluminun pistons.
Ceramic materials are known in the art that have high insulating qualities but are typically fragile and can crack if subjected to forces resulting from differential thermal expansion of adjacent materials. What is needed, to utilize the advantages of aluminum or other low density material and to overcome the thermal stress problem associated with aluminum, is a method of supporting a highly dense and insulative ceramic cap on an aluminum piston in a manner that obviates the difference in coefficient of thermal expansion between aluminum and the ceramic.